This invention relates to microscopic needles having magnetic characteristics that may be used for measurement and testing.
A conventional atomic force microscope (AFM) includes a microscopic needle formed on the free end of a spring-like cantilever, and provides a topographic map of a specimen surface by detecting a repulsive or attractive force acting on the needle. In order to measure or map at high resolution the AFM needle must be sharp, requiring that a tip of the needle have a small radius of curvature.
A scanning tunneling microscope (STM) allows observation of conductive surfaces, by flowing current between a microscopic conductive tip and the conductive surface. This current is very sensitive to a change in distance between the tip and the conductive material, and changes exponentially according to that distance. By scanning the tip along a straight line or flat plane, the surface structure can be observed at an atomic order resolution. Surfaces that can be scanned using STM are typically limited to conductive materials, although thin insulating layers overlying conductive materials can also be analyzed.
A magnetic force microscope (MFM) utilizes a magnetized microscopic tip held on a free end of a cantilever to measure a magnetic field of an object by detecting the magnetic force acting between the object and the tip. A MFM can thus provide two or three-dimensional mapping of various magnetic fields of the object. In addition to structural evaluation of a sample, applicability of MFM to an information storing apparatus which records or reads information directly in a magnetic recording medium using the magnetic field of the tip has been proposed.
Instead of mapping the magnetic field of an object by measuring deflection of a MFM cantilever, the MFM tip can be used to provide a magnetic field to a magnetic sensor for measuring the performance of the sensor. A magnetoresistance sensitivity microscope (MSM) uses a MFM tip to map the sensitivity of a magnetoresistive (MR) sensor, such as may be employed in a disk or tape drive system. The MSM tip may oscillate while a phase lock-in detector measures the electric signal across the sensor at the tip oscillation frequency. Alternatively, the magnetic tip employed in the MFM can also b e used as the field sensor to detect a high frequency magnetic field modulated with the tip oscillation frequency, thus forming a high frequency magnetic force microscope (HFMFM). MSM and HFMFM are discussed in an article entitled xe2x80x9cCharacterization of  greater than 10 Gb/in2 GMR Spin Valve Recording Heads by Novel Magnetic Force Microscope,xe2x80x9d C. X. Qian et al., IEEE Transactions on Magnetics, Sep. 1999, incorporated herein by reference.
In order to obtain high resolution mapping of a MR or GMR sensor, the MFM tip may be located less than 100 nm from the sensor. At that distance, the MFM needle may provide an excessive magnetic field to the sensor that prevents accurate measurement of sensor performance. In addition, the field from the sensor or nearby magnetic elements may redirect or reverse the magnetization of the MFM needle. A conventional MFM needle can be made with a smaller magnetic moment by reducing the amount of magnetic material disposed on the needle. Forming a needle with less magnetic material, however, may exacerbate the problem of reversal of needle magnetization.
The present invention overcomes this dilemma by providing a MFM needle with a more stable magnetic moment. This magnetic moment can optionally be reduced in magnitude without risking an undesirable change in magnetization direction. Such an improved magnetic needle may include a magnetic layer (or layers) that produces the magnetic field and a pinning layer (or layers) that stabilizes the direction of the field-producing magnetic layer. The stabilization can be provided by a pinning mechanism between a ferromagnetic layer and an antiferromagnetic structure. For an embodiment in which the magnetic moment of the needle is reduced, saturation of a MR or GMR sensor by the magnetic field from the needle can be avoided, enhancing resolution in mapping that sensor by MSM. A needle having a smaller yet more stable magnetization can also be disposed closer to a magnetic element being mapped without reversing the needle magnetization. In addition, a needle with a small yet stable magnetic moment can be advantageously employed as a sensor, for instance in HFMFM mapping of high strength magnetic fields that would saturate and/or re-magnetize a conventional needle. Difficulties in producing needles having a desired magnetic direction are also reduced by the present invention. Moreover, advantages of the present invention can work in concert for improving MFM performance in various applications.